10.1 Parental Investment

Robert Trivers first coined the phrase “parental investment” to describe how sex differences in time and energy investment into offspring will influence sexual selection in a species. In his seminal book chapter, “Parental Investment and Sexual Selection,” he sought to unpack how parenting strategies differ between males and females, as well as how these differences can enhance mate retention or improve reproductive success.

Since then, numerous anthropologists and psychologists have sought to define the types of parental investment to better understand how these differences impact social relationships and offspring caregiving. In 1981, Kleiman and Malcolm began to give us terminology that would shape our evolutionary understanding of offspring care.

Direct Care

Direct care consists of activities that influence the immediate survival of offspring. These activities include feeding, bathing, holding, co-sleeping, and playing with offspring. According to John Archer’s paper, “The reality and evolutionary significance of human psychological sex differences,” the female parent in most mammals, including humans, will invest the most in direct care. This makes biological sense. Afterall, before the modern inventions of bottles, formula, and other ways to provide nutrition, it was the female who had to breastfeed, and as a result, hold the offspring more often, too.

Indirect Care

Indirect care consists of activities that influence the long-term survivorship and success of the offspring. These may occur in the presence of offspring, but more often than not, they are activities outside the immediate living space. Indirect care may include resource acquisition, home defense or maintenance, support of parents (usually the mother is the recipient), and elimination of competitors for mating privileges and territory. Males, especially in mammals, usually invest more in the indirect care of offspring. Though in some species, males invest very little at all.

Not All About Sex Differences

Though there are biological and evolutionary reasons for sex differences to exist in parental investment, it is important to note that these are not always hard and fast rules. For example, in many tamarin and marmoset species, females invest very little outside of birth and nursing. Instead, the males will carry, groom, and play with the offspring. Once the young primates are weaned, it is also the male who usually teaches them to secure calories and defend themselves.

While sex differences have certainly been present in our human ancestry, cultural pressures are changing the balance to some degree. The invention of nursing bottles and formula no longer make it a requirement for females to breastfeed. Likewise, changing gender norms in many post-industrial countries suggest that males are under intersexual selection to be more caring and invested. The “sexy father” and “dad bod” trends are examples of how humans may be rewriting the narrative of parental investment.